The intake pressure difference and air density would significantly increase after the significant increase of diesel engine boost ratio. The effects of different boost ratios on intake jet velocity and in-cylinder flow were conducted on a high-boosted single-cylinder diesel engine under two typical speeds through the simulation method. And the intake flow of the highboosted diesel engine was analyzed theoretically. The results show that the intake jet velocity and its distribution as well as the macro and micro velocity field parameters such as in-cylinder swirl ratio and turbulent kinetic energy hardly change with the increase of boost pressure at the same speed. The reason is that the intake jet velocity is affected by the average piston reciprocating movement velocity and the increase of intake air mass depends mainly on the increase of air density.